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January 9, 1879.
W. SPOTTISWOODE, M.A., D.C.L., President, in the Chair.
The Presents received were laid on the table, and thanks ordered for them.
The following Papers were read :
I. “ Researches on the Absorption of the Ultra-Violet Rays of
the Spectrum by Organic Substances.” By W.N. HARTLEY,
Parts I and II.
One of the authors of this paper, Mr. Hartley, having studied the researches of the late Dr. W. A. Miller “On the Photographic Transparency of Various Bodies," &c. (“Phil. Trans.," 1863, I), and of Professor Stokes, “On the Long Spectrum of Electric Light” (“Phil. Trans.," 1863, I), determined to study the action of organic substances on the ultra-violet spectrum. In 1872, the apparatus of Dr. Miller was reconstructed, and some experiments were made which showed that it was capable of some slight improvements. Some time was spent in testing the value of the photographic method of experimenting as compared with that adopted by Professor Stokes, and preference was eventually given to the former, or rather to a combination of both methods, since occasional use was made of a focussing screen either of uranium glass or of white paper steeped in æsculine solution and dried.
soon apparent that a wide field of investigation was opened, and, with the assistance of Mr. Huntington, a systematic course of examination of organic compounds was commenced at the beginning of the present year. In January, 1878, M. Soret published his “Recherches sur l'Absorption des Rayons Ultra-Violet
diverses substances” (“ Archives des Sciences Physiques et Naturelies, Genève"), which includes the examination of many inorganic and some organic compounds. Though this is a work of very great interest, it does not touch upon the subject of the present investigation, namely, the connexion between chemical constitution and diactinic quality. M. Soret uses a spectro
scope of his invention which receives the ultra-violet rays upon a fluorescent eye-piece, and so renders them visible.
The Apparatus.—This consisted of a spectroscope attached to a photographic camera, the prism and lenses being of quartz. The electric light consisted of sparks of great intensity passed between metallic electrodes. To produce the sparks an induction coil, capable of giving a 7-inch spark in air, was excited by five cells of Groves' battery. A Leyden jar was interposed between the coil and the electrodes, each surface of the foil measuring 72 square inches. The electrodes found to answer best were points of nickel wire, containing a trace of copper. Cells of glass, with quartz sides, were used for holding liquids under examination. These cells were placed behind the slit of the spectroscope, the spark passing in front; volatile liquids were thus prevented from taking fire, and a certain loss of light was avoided. No condensing lens was used in front of the slit, because occasionally it was found convenient to employ an amalgam, containing zinc, cadmium, aluminium, and magnesium, dissolved in mercury, in conjunction with a point of iron, and under these circumstances volatilised mercury would condense on the lens.
The Photographic Process. It was found by experiment that a wet collodion process, as used by Dr. Miller, was disadvantageons for several reasons, and therefore dry plates were used. A preference was given to gelatine pellicle plates, containing bromide of silver. They are quite sufficiently sensitive, give a very finely defined picture, and do not necessitate a varnishing process. The exposure has generally been about 10 seconds, but on certain occasions plates have been in the camera for an hour and a half. We have found no difficulty in obtaining a constant stream of sparks, giving a steady light for three-quarters of an hour without intermission.
The Measurement of Absorption-Bands, &c. In order to measure the degree of absorption exercised by different substances the example of M. Soret has been followed, and the lines of cadmium have been taken for the purpose.
M. Mascart has measured the wave-length of these lines both for the visible and the ultra-violet
rays. Sometimes measurements on the scale of wavelength have been adopted, but in other cases it has been found more convenient to make use of spectra as photographed. Photographs of different metallic spectra employed are presented. The lines of cadmium are distinguished by the numbers assigned to them by M. Mascart. A comparison is also given of the relative extent of the visible and ultra-violet rays after passage through a prism.
The prism was placed at the angle of minimum deviation for the sodiam line D.
The various parts of the apparatus are screwed down so as to be immovable after a proper adjustment.
The Examination of Organic Substances: Dr. Miller failed to trace any connexion between the chemical complexity of a substance and its actinic absorption.
With the view of ascertaining whether any such connexion existed an examination was made of the normal alcohols, the normal fatty acids, and a series of ethereal salts. Great trouble was occasioned by the interference of minute traces of otherwise undetected impurities, the presence of which was often unaccountable. Four diagrams, showing the relative transparency of different substances, illustrate this part of the paper, and from the results obtained the following conclusions have been drawn..
(1.) The normal alcohols of the series C,H2n+OH are remarkable for transparency to the ultra-violet rays of the spectrum, pure methylic alcohol being as nearly so as water.
(2.) The normal fatty acids exhibit a greater absorption of the more refrangible rays of the ultra-violet spectrum than the normal alcohols containing the same number of carbon-atoms.
(3.) There is an increased absorption of the more refrangible rays corresponding to each increment of CH, in the molecule of the alcohols and acids.
(4.) Like the alcohols and acids, the ethereal salts derived from them are highly transparent to the ultra-violet rays, and do not exhibit absorption-bands.
In order to ascertain whether isomeric bodies exhibited similar or identical absorption spectra a series of benzene derivatives was examined.
From the great absorptive power of this class of substances it was found necessary to use very dilute solutions even though the cells holding the liquids were not more than 0.75 inch in thickness. Curves were plotted by taking the proportions of substances in solution as ordinates, and the position of absorption-bands as abscissæ, and these curves are highly characteristie features of very many compounds. About twenty diagrams have thus been made.
The following is a summary of the chief points of interest appertaining to benzene and its derivatives.
(1.) Benzene, and the hydrocarbons, the phenols, acids, and amines derived therefrom, are remarkable firstly, for their powerful absorp
tion of the ultra-violet rays; secondly, for the absorption-bands made • visible by dissolving them in water or alcohol, and diluting; and
thirdly, for the extraordinary intensity of these absorption-bands, that is to say, their power of resisting dilution. (2.) Isomeric bodies, containing the benzene nucleus, exhibit widely different spectra, inasmuch as their absorption-bands vary in position and in intensity.
(3.) The photographic absorption spectra can be employed as a means of identifying organic substances, and as a most delicate test of their purity. The curves obtained by co-ordinating the extent of dilution with the position of the rays of the spectrum absorbed by the solution form a strongly marked and often a highly characteristic feature of many organic compounds.
There is a curious feature in connexion with the position of the absorption bands; at the less refrangible end they either begin at line 12 Cd or line 17 Cd, and those which begin at 12 end a little beyond 17.
No naphthalene or anthracene derivatives have yet been examined, and very few substances of unknown constitution-hence most interesting results may be anticipated from a continuation of this research, and this contribution must be accepted rather as a bare commencement of the subject than its conclusion.
II. “On the Electromagnetic Theory of the Reflection and Re
fraction of Light.” By GEORGE FRANCIS FITZGERALD, M.A., Fellow of Trinity College, Dublin. Communicated by G. J. STONEY, M.A., F.R.S., Secretary of the Queen's University, Ireland. Received October 26, 1878.
(Abstract.) The media, at whose surfaces reflection and refraction are supposed to take place, are assumed to be non-conductors, and isotropic as regards magnetic inductive capacity. Some reasons are advanced why the results should apply at least approximately to conductors. In the first part of the paper the media are not assumed to be isotropic as regards electrostatic inductive capacity, so that the results are generally applicable to reflection and refraction at the surfaces of crystals. I use the expressions given by Professor J. Clerk Maxwell in his "Electricity and Magnetism," vol. ii, Part IV, chap. 11, for the electrostatic and electrokinetic energy of such media. By assuming three quantities, g, y, and g, such that, t representing time,
dt' dt de and are the components of the magnetic force at any point, I have thrown these expressions for the electrostatic and electrokinetic energy of a medium into the same forms as M'Cullagh assumed to represent the potential and kinetic energy of the ether, in “ An Essay towards a Dynamical Theory of Crystalline Reflection and Refraction," pub